1. Field of the Invention
The present invention relates to an optical amplifying and repeating method and an optical amplifying and repeating system, and, in particular, to an optical amplifying and repeating method and an optical amplifying and repeating system suitable to monitor a loss distribution in an optical transmission line in a longitudinal direction by means of OTDR (Optical Time Domain Reflectometry; optical pulse test).
2. Description of the Related Art
As a method of monitoring a loss distribution in a longitudinal direction of an optical fiber transmission line, OTDR including coherent OTDR is common (for example, see patent document 1 or non-patent document 1).
In a system including an optical amplifying and repeating unit in an optical transmission line, an optical isolator is required in an optical amplifier in terms of avoiding multipath reflection. However, since backscattering light is blocked by the optical isolator in the optical amplifier, it is not possible to carry out OTDR in a condition in which the optical amplifier having the optical isolator built therein is inserted.
FIG. 1 shows one example of a configuration of the optical amplifying and repeating unit system in the prior art. In the figure, a wavelength λa, transmitted from an A station, and traveling in an A direction (a direction in which light travels from the A station to a B station) from the A station in a transmission line A (a transmission line in which light travels from the A station to the B station), passes through an optical coupler 10, an optical amplifier 11 and an optical coupler 12, travels in the A direction in the transmission line A, and then, is transmitted to the station B. A wavelength λb, transmitted from the B station and traveling in the B direction (a direction in which light travels from the B station to the A station) from the B station in a transmission line B (a transmission line in which light travels from the B station to the A station), passes through an optical coupler 13, an optical amplifier 14 and an optical coupler 15, travels in the B direction in the transmission line B, and then, is transmitted to the station A. The above-mentioned optical coupler 10, optical amplifier 11, optical coupler 12, optical coupler 13, optical amplifier 14 and optical coupler 15 are disposed on an optical amplifying and repeating unit.
Backscattering light of the wavelength λa is provided to the light transmission line B from the coupler 12 and via the coupler 13, then travels in the B direction there, and is monitored by an OTDR measurement unit 17 provided in the A station. Backscattering light of the wavelength λb is provided to the light transmission line A from the coupler 15 via the coupler 10, then travels in the A direction there, and is monitored by an OTDR measurement unit, not shown, provided in the B station.
In OTDR known from patent documents 2, 3, 4, 5 or such, as illustrated with reference to FIG. 1, only a loss distribution in a longitudinal direction of the optical transmission line A can be monitored from the A station, and, it is not possible to monitor a loss distribution in a longitudinal direction of the optical transmission line B (opposite line) from the A station. Similarly, only the loss distribution in the longitudinal direction of the optical transmission line B can be monitored from the B station and, it is not possible to monitor the loss distribution in the longitudinal direction of the optical transmission line A (opposite line) from the B station.
Patent document 1: Japanese Laid-open Patent Application No. 9-236513;
Patent document 2: Japanese Laid-open Patent Application No. 10-229366;
Patent document 3: Japanese Laid-open Patent Application No. 9-116502;
Patent document 4: Japanese Laid-open Patent Application No. 2001-505312;
patent document 5: Japanese Laid-open Patent Application No. 2001-502422; and
Non-patent document 1: Submarine Cable Network Systems, written by Shigeyuki Akiba and Naruhito Nishi together, issued by NTT Quality Co., Ltd.
If the loss distributions in both A and B directions can be monitored from the A station (or the B station), it is possible to reduce facilities required for a maintenance of the optical transmission lines and a manpower concerning the maintenance. Also, for a system having a long repeating span of optical amplifying and repeating unit, a high dynamic range is required for the OTDR measurement. If the loss distributions in both directions of the transmission lines A and B can be monitored from both A and B stations, it is possible to achieve the dynamic range double that of the prior art by monitoring the half span of the transmission line A and the transmission line B on the side of A from the A station while monitoring the half span of the transmission line A and the transmission line B on the side of B from the B station. However, in the prior art, the above-mentioned improvement of the maintenance or improvement of the dynamic range cannot be achieved.